With the advanced automatization of equipment arranged in the drive train of motor vehicles, there is an increased necessity to provide a coordinated control thereof. For this purpose, operating parameters of the motor vehicle, especially of the equipment arranged in the drive train, are read into a control apparatus, evaluated and corresponding desired quantities are made available for the equipment. Automated equipment includes, for example, the following: a vehicle engine, a clutch or an automatic transmission. In the case of an automatic transmission, it has been shown to be especially difficult to adapt a wheel torque, which is applied to the vehicle wheels, to the actual requirements because an instantaneous vehicle load is not considered or is only considered inadequately in accordance with the conventional methods. The deficient consideration is based essentially on the difficulty to be able to determine with adequate accuracy the instantaneous vehicle load.
It is an object of the invention to provide a much more comfortable driving performance of the vehicle during the change of the transmission ratio. For example, a drop in torque is avoided at the start of uphill travel or with additional loading in that the transmission ratio of the transmission is set in dependence upon the instantaneous vehicle load.
The method of the invention is for determining a transmission ratio (i) for an automatic transmission mounted in the drive train of a motor vehicle. The method includes the steps of: providing a control apparatus wherein operating parameters of the motor vehicle are made available to determine the transmission ratio (i); and, determining the transmission ratio (i) of the automatic transmission in dependence upon an instantaneous load of the motor vehicle.
It has been shown to be especially advantageous to include the instantaneous vehicle load in a characteristic line for a throttle flap position. From the characteristic line, the required transmission ratio is then read out. The characteristic line, which is determined in this manner, can be corrected with the aid of suitable factors in an advantageous manner. In this way, a maximum acceleration can be ensured to the driver of the vehicle in each situation by depressing the accelerator pedal all the way down. To determine the instantaneous vehicle load, an uphill travel and a downhill travel, the additional loading and the total running resistance must be determined in order to determine from all of the above the transmission ratio to be selected. With the aid of the above-mentioned quantities, an acceleration of the vehicle can also be determined which is an index for the instantaneous vehicle load. The characteristic lines for the transmission ratio can therefore be set in dependence upon the acceleration.
In a preferred embodiment of the method, the acceleration in the form of a difference acceleration flows into the determination of the characteristic lines. The difference acceleration results from an expected and an actual acceleration. Advantageously, the expected acceleration is first averaged and thereafter normalized.
An especially exact prediction of the acceleration to be expected can be obtained with the aid of operating parameters such as a dynamic wheel radius, a transverse acceleration, a resulting wheel torque and a total running resistance torque. The total running resistance torque can be determined in dependence upon a vehicle speed from a characteristic line which is taken up for an unloaded vehicle in a plane; whereas, the resulting wheel torque is advantageously determined in dependence upon an output torque of a transmission output shaft, a rear axis ratio and a wheel rotation torque. The above-mentioned quantities can preferably be determined from a gradient of a wheel rpm, a mass inertia of the wheels, a number of the wheels, a converter amplification and an engine rotation torque. The above-mentioned procedures supply a value for the instantaneous vehicle load which comes especially close to the real value.